A dielectric multilayer filter, a color filter with dispersed colorant or the like is widely employed as a filter transmitting selectively a specified wavelength. As means for performing easier the same function as such, a wavelength-selective filter with a diffractive grating is proposed. The wavelength-selective filter comprises cyclic concave/convex portions formed on a transparent substrate. This wavelength selective filter can provide high transmission characteristics with respect to a specified wavelength when the phase difference of incident light to a concave portion and a convex portion of the filter is an integral multiple of the specific wavelength.
Further, a diffractive dichloric filter (wavelength-selective filter) utilizing this technique is disclosed in, for example, JP-A-2000-348366. In the diffractive dichloric filter of this patent document, an optical head device comprising two kinds of semiconductor lasers as light sources which emit wavelengths of 660 nm and 790 nm, is used wherein when the phase difference of incident light produced by the steps of concave and convex is about two time as much as 660 nm, light having a wavelength of 660 nm is transmitted and light having a wavelength of 790 nm is shielded by diffraction (hereinbelow, referred to as “diffraction-shield”), hence, it functions as the diffraction-shielding filter.